Added publishing functionality to Pubsubhubbub

Added the functionality of sending the alert to Pubsubhubbub.

Right now we have all the valid info only about pubsubhubbub . Will add a broker table later as and when more brokers are available.
If you wouldn't hand-craft the form, but use SQLFORM.factory, then the widgets would be inferred from the validators ;) and validation would be included in the process. That way, you wouldn't need this case discrimination, and in fact (see previous comment) I think this isn't an if-else. It should be a MultiSelect allowing multiple channels to be selected simultaneously.